1. Field of the invention
This invention relates to a control system for an automatic transmission for vehicles, and, more particularly, to an automatic transmission control system in which a timing at which two friction coupling elements are alternately locked and unlocked to provide a specific gear is determined based on a turbine speed.
2. Description of Related Art
Typically, an automatic transmission for an automobile has a torque converter and a transmission gear mechanism including a plurality of friction coupling elements, such as clutches and brakes, which are selectively locked and unlocked to place automatically the automatic transmission into desired gears according to driving conditions. In such an automatic transmission, a target gear to which the automatic transmission is shifted is ordinarily determined according to driving conditions including a vehicle speed and an engine load or throttle opening. Specifically, the target gear is determined based on predetermined gear shift patterns so as to become higher with an increase in vehicle speed and/or a decrease in engine load. Selectively locking and unlocking the frictional coupling elements is controlled by means of a hydraulic control circuit such as to place the transmission gear mechanism in the target gear. In this instance, in order to prevent or significantly reduce shift shock, locking or unlocking an friction coupling element is controlled not to be momentarily achieved but to progress gradually with slippage so that the speed of a turbine of the torque converter (which is hereafter referred to as the turbine speed) smoothly changes to a speed to be attained at completion of a gear shift following a target speed during the gear shift. Such a friction coupling element locking/unlocking control is known from, for example, Japanese Unexamined Patent Publication No. 1-1 99083. Further, during, for example, a specific gear shift which is provided as result of alternately locking and unlocking specific two friction coupling elements, either one of the two friction coupling elements is locked or unlocked ahead of unlocking or locking the other one to provide an inertia phase. In the inertia phase, hydraulic pressure for locking the friction coupling element is feedback controlled to rise gradually such that the turbine speed drops to a specified turbine speed when the gear is up-shifted or rises to a specified turbine speed when the gear is down-shifted and the two friction coupling element is completely locked and unlocked timely when the specified turbine speed is attained or almost attained. This control prevents a coincidental lock or interlock of the two friction coupling elements which result in an occurrence of shift shock.
In order to cause timely the alternate lock and unlock operation of the two friction coupling elements to provide the specific gear shift, it is necessary to detect a turbine speed. If a meters associated to turbine speed, such as a turbine speed sensor, breaks down, it is impossible to determine a timing at which locking or unlocking the other friction coupling element is to commence, as a result of which an untimely lock or unlock of the other friction coupling element occurs with an adverse effect of causing interlock or shift shock and an abrupt rise in engine speed.